THE LIANA HABIT 93 



and are practically unmodified except in their sensitiveness to con- 

 tact. A comparable condition is found in various species of Clematis 

 where the stalks (petiohdes) of the leaflets serve as tendrils. In the 

 sweet pea and some other members of the legume family some of the 

 terminal leaflets of the compound leaves are modified into tendrils. 

 In Smilax the tendrils have the position of stipules. Since stipules 

 are normally absent in monocotyledons, however, these tendrils are 

 probably outgrowths from the leaf base. In the grape and other 

 members of the Vitaceae the tendrils are modified branches. In the 

 squash and other members of the Cucurbitacese the origin of the 

 tendrils is somewhat obscure, but it seems likely that, when they are 

 simple, they are to be considered as modified leaves, and when they 

 are branched, they represent shoots bearing leaves. Tendrils modi- 

 fied from roots are not common but are found in a few cases, as, 

 for example, the orchid. Vanilla. In a few plants, especially some 

 species of Ampelopsis, the tendrils do not twine about the support 

 but form adhesive disks at their tips which cling to a solid surface 

 with great tenacity. 



58. Root Climbers and Leaners. — Climbing by means of roots is 

 not very common among plants of the temperate zones but there are 

 many tropical plants that climb by this means. Familiar examples 

 in the north temperate zone are the poison i\y (Fig. 37) and some 

 varieties of the Virginia creeper (Fig. 38). These produce verv large 

 numbers of adventitious roots along their stems and these roots grow 

 horizontally about the hosts as well as into the cracks and softer 

 parts of the bark. Such success as these plants have in maintaining 

 their positions is to be attributed to the large number of anchoring 

 organs rather than to the eflSciejicy of individual roots. 



Some plants, such as the bittersweet nightshade {Solanmn dul- 

 camara) do not have sufficient mechanical tissue to hold themselves 

 erect and yet have no special means for climbing. Such plants often 

 lean upon neighboring plants and may be spoken of as leaners. 

 There are many plants which are prostrate when unsurrounded by 

 other plants but which become leaners when growing in dense 

 vegetation. Some leaners are kept from slipping from their supports 

 by the presence of spines or prickles as in roses and blackberries. 

 These are especially effective if they point downward as in the species 

 of bedstraw {Galium). 



59. The Liana Habit.— The chief advantage of the liana habit is 

 that it enables a plant to get its leaves^ up into the light with- 

 out constructing a large amount of mechanical tissue. The chief 



